This invention relates to a body implantable lead bearing an electrode for connecting a living body organ to an electrical device, and more particularly to the combination of such a lead with an improved stiffening stylet. Notwithstanding its various uses, this invention will be described as an endocardial pacing and sensing lead for connecting a pacemaker pulse generator to cardiac tissue.
Endocardial pacing and sensing leads of the types shown in U.S. Pat. No. 3,128,418 for example, comprise one or more lengths of hollow, coiled wire conductor encased within a suitable insulating material, such as silicone rubber, that is substantially inert to body fluids and tissues, a hollow connector pin attached to the proximal end of each conductor, and an electrically conductive electrode at the distal end of the conductor adapted to be placed in contact with the endocardium of a patient. A lumen extends through each pin and the corresponding length of coiled wire conductor to the electrode at the distal end thereof and receives a stiffening stylet of cylindrical stainless steel wire for imparting stiffness to the lead to facilitate its advancement through the venous system of the patient and into the apex of the right ventricle. With the stylet removed from the lead, the lead is very flexible and difficult to so advance. Further details of the construction and utility of such an endocardial pacing lead may be obtained by reference to the aforementioned U.S. Pat. No. 3,348,548.
The conventional endocardial pacing lead employs a relatively blunt tip electrode at its distal end that is adapted to be lodged in the trabeculae at the apex of the right ventricle. An improved endocardial pacing lead employing a rigid, electrically conductive helix with a sharp tip at the distal end of the lead adapted to be screwed into the endocardium is disclosed in copending U.S. Pat. No. 3,974,834 issued Aug. 17, 1976 entitled BODY IMPLANTABLE LEAD and assigned to Medtronic, Inc. The improved lead can be lodged in and permanently secured to or removed from body tissue without the use of complex electrode advancement mechanisms or bulky sleeves or catheter introducers to protect the patient's veins and tricuspid valve from snagging on the sharp tip of the helix. Placement of this improved body implantable lead is effected through the employment of a shroud of resilient non-conductive material extending from the silicon rubber casing of the lead through the length of the helix that is adapted to retract upon contact with tissue as the helical electrode is screwed therein. One or more conventional stylets of stainless steel wire are employed to stiffen the improved body implantable lead during the introduction and placement thereof.
The body implantable lead of the present invention incorporates an improved stylet or stylets that increase their effectiveness in the placement of either the conventional, endocardial leads of the type disclosed in the aforementioned U.S. Pat. No. 3,348,548 or the improved helical electrode lead. One of the features of the present invention is the provision of a stylet adapted to stretch and lengthen the helical wire conductor beyond its relaxed length to increase the rigidity of the lead and the ability of the lead to transmit torque from its proximal end to its distal end.